Mass spectrometer and mass filters therefor

ABSTRACT

A mass filter apparatus for filtering a beam of ions is described. The apparatus comprises an ion beam source and first and second mass filter stages in series to receive the ion beam. A vacuum system maintains the first and second filter stages at substantially the same operating pressure, below 10 −3  torr. The first mass filter stage transmits only ions having a sub-range of mass-to-charge ratios including a selected mass-to-charge ratio. The second filter transmits only ions of the selected mass-to-charge ratio. The second mass filter can achieve high accuracy detection without being subjected to problems such as build-up of material on quadrupole rods, resulting in a distorted electric field close to the rods. The first mass filter acts as a coarse filter, typically transmitting 1% of ions received from the ion source. Thus, the detection accuracy and lifetime of mass spectrometers embodying this invention are greatly improved.

PRIOR APPLICATIONS

This application claims benefit of Patent Cooperation Treaty ApplicationNumber PCT/GB03/02041, filed May 13, 2003, which claims priority fromGreat Britain Application Number 0210930.4, filed May 13, 2002.

TECHNICAL FIELD

This invention relates to a method and apparatus for improvingoperational characteristics of mass spectrometers.

The invention is described herein with reference to quadrupole massfilter arrangements, but is not limited to such apparatus.

BACKGROUND

Quadrupole, or multipole mass filters are known in the mass spectroscopyart and operate to transmit ions having a mass/charge ratio which liewithin a stable operating region. The size of the stable operatingregion is governed by the geometry of quadrupole rods, and themagnitudes of DC and RF voltages (including the RF voltage's frequency)applied to the rods, amongst other factors. Thus, the transmitted ionscan have a range of mass/charge ratios depending on the size of thestable operating region. The transmission characteristics of the filter,and hence the range of mass/charge ratios within the transmitted, orfiltered ion beam, can be reduced by reducing the stable operatingregion's size. Rejected ions are not transmitted to the spectrometer'soutput or detector.

A substantial proportion of the rejected ions strike the quadrupole rodssputtering material from, and/or depositing dielectric material onto therods. A large amount of deposition can occur over time, particularlywhen a spectrometer is used to analyse masses of particles withinrelatively intense ion beams. Deposited material can result in areas ofthe rod's surface becoming partially or completely insulating, or havinga different electrical work function. Thus, the material deposited onthe rods affects the characteristics of the electric field associatedwith the voltages applied to the rods. Ultimately, the depositedmaterial changes the electric field strength near the surface of therods.

A further problem, known as the space charge effect, occurs whenanalysing relatively intense ion beams. As the intense ion beam entersthe quadrupole mass filter the electric field associated with thevoltages applied to the quadrupole rods is distorted. This distortion ofthe field is due to the presence of the charged particles in the ionbeam. The electric field distortions occur in the vicinity of the ionsin the beam.

Quadrupole mass filters are seriously affected by these problems,particularly when a spectrometer comprising such filters operates at ahigh mass resolution. Very onerous demands on the precision with whichthe electric field is maintained are required for high resolution massspectrometry. Furthermore, at high resolving powers, the stabletrajectories of ions through the filter pass very close to the rods forrelatively long distances in the filter. Therefore, the trajectoriespass very close to the deposited dielectric material, and hence within aregion of the electric field suffering from distortions.

Also, the resolving power of a spectrometer is approximatelyproportional to the square of time spent in the filter by the ions.Thus, a desired resolution may only be achieved if the ions spendsufficient time in the filter; the longer the ions spend in the filter,the greater the resolution obtained. It is usual to decelerate the ionsto very low energies (typically 2 ev) to maximise time spent in thefilter, and hence increase resolving power of the spectrometer. Thespace charge effect is high for such a slow ion beam, and thisexacerbates the problems associated with distorted electrical fieldscaused by the space charge effect. Thus, presently there is a compromisebetween the space charge effect, ion beam energy and spectrometer massresolution.

A mass filter having a distorted electric field caused by the problemsdescribed above can have a considerably reduced mass resolving power ortransmission. In the worst case, the spectrometer is rendered useless.The problems are exacerbated over time as more dielectric material isdeposited on the rods. The accumulation of material tends to be unevenwith more material deposited close to the entrance of the filter sincemost ions are rejected on entry into the filter. When the spectrometer'sperformance falls below a tolerable level it is necessary to replace orrefurbish the mass filter at considerable cost.

U.S. Pat. No. 3,129,327 discloses auxiliary electrode rods which aredriven only by AC voltages to improve transmission into a second set ofrods which act as a mass filter; the auxiliary electrodes act as an ionguide.

U.S. Pat. No. 4,963,736 discloses a rod set operating with substantiallyno DC voltage and at an elevated pressure. Thus, the filter act as apressurised ion guide which has high transmission properties due tocollision focussing.

U.S. Pat. No. 6,140,638 discloses a mass filter comprising a firstfilter operating as a collision/reaction cell and at an elevated gaspressure with respect to a second filter. The apparatus disclosed aimsto reduce isobaric interferences by transmitting ions through acollision cell to reject intermediate ions which would otherwise causeisobaric interferences.

U.S. Pat. No. 6,340,814 discloses a spectrometer comprising two filtersoperating with similar mass resolution to improve the resolution of thewhole device. When the two filters are coupled to one another, a higherresolution is achieved compared to the resolving power of each filterseparately.

EP1114437 discloses a method and apparatus for removing ions from an ionbeam to reduce the gas load on the collision cell which serves tominimise the formation, or reformation, of unwanted artefact ions in thecollision cell.

None of these systems propose a solution to the problems describedabove.

SUMMARY OF THE INVENTION

It is an aim of the present invention to ameliorate the problemsassociated with the prior art. In their broadest form, embodiments ofthe invention reside in a mass spectrometer which comprises a multiplemass filter stage. In one of the mass filters a large proportion ofunwanted ions are removed from the ion beam.

More precisely, there is provided a mass filter apparatus for filteringa beam of ions having mass/charge ratios in a range of mass/chargeratios to transmit ions of a selected mass/charge ratio in the saidrange, comprising an ion beam source for emitting the ion beam, firstand second mass filter stages in series to receive the beam from thebeam source, and a vacuum system for maintaining at least the secondfilter stage at an operating pressure below 10⁻³ torr, wherein saidvacuum system is arranged to maintain both the first and second filterstages at operating pressures below 10⁻³ torr, the first mass filterstage is arranged for transmitting only ions having a sub-range ofmass/charge ratios which includes the selected mass/charge ratio, andthe second mass filter is arranged for transmitting only ions of thesaid selected mass/charge ratio.

Also, there is provided a method for filtering a beam of ions havingmass/charge ratios within a range of mass/charge ratios to transmit ionsof a selected mass/charge ratio in the said range, the methodcomprising; emitting the ion beam from a beam source into a first massfilter stage; transmitting through the first mass filter stage only ionshaving a sub-range of mass/charge ratios which includes the selectedmass/charge ratio; and transmitting through a second mass filter stagein series with the first mass filter only ions having the selectedmass/charge ratio, wherein the first and second filter stages operate atpressures below 10⁻³ torr.

Furthermore, there is provided a method for filtering ions with a givenmass/charge ratio from a beam of ions having an array of mass/chargeratios, in a mass spectrometer comprising an ion beam source foremitting the ion beam, a detector or output for detecting ortransmitting the filtered ions, and a plurality of mass filters disposedin series between the beam source and the detector or output, thefilters having the same operating pressures at or below 10⁻³ torr, themethod comprising; emitting the ion beam from a beam source into a firstmass filter, transmitting only ions having a range of mass/charge ratioswhich includes the mass/charge ratio of the filtered ions from a firstmass filter, and transmitting only the filtered ions from a second massfilter, disposed between the first mass filter and the detector oroutput.

Yet further, there is provided a method for producing a mass spectrum ofa beam ions having mass/charge ratios within a range of mass/chargeratios, comprising; emitting the ion beam from a beam source into afirst mass filter stage, transmitting only ions having a sub-range ofmass/charge ratios which includes a selected mass/charge ratio throughthe first mass filter, transmitting only ions having the selectedmass/charge ratio through a second mass filter in series with the firstmass filter to a detector for detecting any ions having the selectedmass/charge ratio, controlling at least the second filter stage so thatthe mass/charge ratio of transmitted ions is scanned over a scannedrange, and detecting the number of ions transmitted by the second filterstage at any given mass/charge ratio to provide a mass spectrum, whereinthe first and second filter stages operate at pressures below 10⁻³ torr.

Yet still further, there is provided a method of improving the resolvingpower of a mass spectrometer, comprising; emitting an ion beam from abeam source into a first and second mass filter stages in series, theions in the beam having mass/charge ratios within a range of mass/chargeratios; transmitting through the first mass filter stage only ionshaving a sub-range of mass/charge ratios which includes a selectedmass/charge ratio; receiving only ions in said sub-range at the secondfilter stage; transmitting through a second mass filter stage only ionshaving the selected mass/charge ratio, whereby the second filter stagecan operate with reduced ion beam current.

Further still, there is provided a method for reducing the deposition ofmaterial on multipole elements of a primary resolving filter of a massspectrometer, comprising emitting an ion beam from a beam source into afirst mass filter stage, the ions in the beam having mass/charge ratioswithin a range of mass/charge ratios, transmitting through the firstmass filter stage only ions having a sub-range of mass/charge ratioswhich includes a selected mass/charge ratio, receiving only ions in saidsub-range at a second filter stage in series with said first filterstage, said second filter stage constituting said primary resolvingfilter, and transmitting through the second filter stage only ionshaving a selected mass/charge ratio within the sub-range, therebyreducing the number of ions rejected in said primary resolving filter.

Embodiments of the present invention have an advantage of operating withhigh resolution over much longer periods, compared to previous systems.A coarse filter removes the majority of unwanted ions from the ion beamand is arranged to operate with a relatively high band pass comparedwith a fine filter. Thus, the problems described above associated withthe prior art can be reduced for the filters and the accuracy of thefilter can be improved.

The operational procedures for an apparatus or method embodying theinvention can be greatly simplified with respect to devices that utilisecollision or reaction cells in the filter stages of the spectrometer.The only gases likely to be present in the filters of the devicesembodying the present invention are very low level traces of residualgases such water vapour, CO₂, or Ar which are mostly derived from theion source, residue in the filter or purge gas. Traces of these gases atpartial pressures below 10⁻³ torr in a typical filter are insufficientto cause any significant number of reactions with the ions being passingthrough the filter.

Devices and methods embodying the invention also have the advantage ofless problematic operation, especially at high resolving powers, andwhen compared to spectrometers comprising collision or reaction cells.The spectrum produced by devices utilising collision or reaction cellscan include unwanted peaks derived from reacted ions. The transmissionof ions through the reaction/collision cell is reduced by the collisionsor reactions, and so the sensitivity of the device is affected. Thecomplexity to such device's operation is high because of the controlsnecessary for operating the reaction/collision cells. Also, a highdegree of knowledge in ion collision chemistry is required by theoperator to ensure the correct gas is used, otherwise the requiredreaction does not occur and the spectral results can be misleading oruseless. Embodiments of the present invention operate at pressures wherereactions or collisions are very unlikely to occur in the filter stage.

As described above the filters operate at a high vacuum of 10⁻³ torr, orless, at which pressures the density of gas molecules in the filter isat such a level that the likelihood of reactions or collisions takingplace between the ions in the beam and any residual gas in the filter isvery low or none existent. This has a further advantage that hightransmission coefficients through the filters for the desirable ions canbe achieved (and hence improvements to the sensitivity of thespectrometer is also improved).

Such advantages are particularly desirable for high resolution massspectrometers. Such systems might typically operate at 10⁻⁶ torr, atwhich pressure, if there are any collisions and/or reactions of ionswith the gas in the filter they have virtually no affect on the ion beamintensity or resulting spectrums. Thus, advantageously, embodiments ofthe present invention can operate at extremely high resolving powers andhigh beam intensities.

Also, a single vacuum pump can be used to maintain the vacuum in allfilter stages, thus further simplifying the system.

Another advantage is achieved by removing a majority of ions from theion beam in the first filter stage, and hence reducing the beam currentin the second filter stage. Thus, the amount of material deposited onthe second filter stage's elements is greatly reduced, allowing thesecond filter stage to operate with very high resolving powers for muchlonger periods of time. The time between service intervals can thereforebe increased, increasing the time in which the spectrometer isoperational and reducing costs. The second filter stage can also operateat very high resolving powers since the electric field characteristicsin the filter remain substantially constant because of the much reduceddeposition of dielectric material in the filter. The space charge effectcan be calculated with a high degree of accuracy and compensated for.The space charge effect is much lower due to reduced beam current, thusfurther improving the resolving powers of the device.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS

Embodiments of the present invention are now described, by way ofexample and with reference to the accompanying drawings, in which:

FIG. 1 is a highly schematic representation of an embodiment of thepresent invention; and

FIG. 2 is a highly schematic representation of another embodiment of thepresent invention.

Referring to FIG. 1, a mass spectrometer 10 embodying the presentinvention is shown. The spectrometer comprises an ion beam source 12 anda detector 14. Disposed between the ion source and the detector are twovacuum chambers 16 and 18 respectively. Each chamber is maintained at ahigh level of vacuum by vacuum pumps 20 and 22 respectively. Vacuum pump24 is used to evacuate the ion beam source beam chamber 12, if required.Mass filters 30 and 32 are each disposed in chambers 16 and 18respectively. The filters are disposed in series relative to one anotherand the ion beam source. Thus, the ion beam passes first through onefilter and then the other before striking the detector, or beingemitting from an output (not shown). Quadrupole rods 34 and 36 arearranged to influence the ions in the ion beam passing through the massfilters 30 and 32 respectively.

For the purpose of this description, the filter 30 closest to the beamsource chamber 12 is termed a “sacrificial filter”. The filter 32closest to the detector 14 is termed the “analysis filter”.

The sacrificial filter operates at a lower resolving power and providesa more broad stability region than the analysis filter. The stabilityregion of the sacrificial filter is set so that most of the massspectrum of ions entering the filter is rejected. Put another way, thesacrificial filter acts to pre-filter the beam before it enters theanalysis filter.

A high proportion of rejected ions strike the quadrupole rods of thesacrificial filter causing deposition thereon, but because the filterhas a relatively broad stability region, any distortions of the electricfield caused by such deposits in the filter 30 do not cause rejection ofions of the required mass/charge ratio. Thus, a large amount of unwantedmaterial is removed from the ion beam before it enters the analysisfilter, whilst substantially all ions of the required mass/charge ratioare transmitted to the analysis filter.

In addition, the high intensity ion beam entering the sacrificial filter30 can distort the electric field by the space charge effect. The broadstability region of the sacrificial filter continues to operate so thatsubstantial all the ions of the required mass/charge ratio aretransmitted to the analysis filter. However, advantageously, the spacecharge effect in the analysis filter 32 is greatly reduced due to thereduced ion beam intensity or ion current, the majority of ions in thebeam having been rejected in the sacrificial filter.

Furthermore, the sacrificial filter can operate at higher ion energies,relative to the analysis filter. The ions can be decelerated beforeentering the analysis filter to roughly ⅕ the energy with which theytransit the sacrificial filter. The sacrificial filter can be arrangedto remove most of the unwanted ion beam current at the increased beamenergy.

Also, the transmission of ions through the sacrificial filter isrelatively high because of the high ion energy. In a preferredembodiment, the sacrificial filter typically removes 99.9% of the ioncurrent. Put another way, 0.1% of ions in the ion beam are transmittedby the sacrificial filter. More preferably the sacrificial filteroperates with a 0.01% transmission factor for very high resolutionapplications. As a result, the space charge effect and deposition ofunwanted material on the analysis filter is reduced by a factor, in theorder of 99.99%. Embodiments of the invention are particular effectivewhere ion currents of 100 nA or more are present and when a resolutionof 0.1 atomic mass units (amu) is required. At very high resolution(that is in the order of 0.02 amu) embodiments of the invention areextremely effectual.

The analysis filter is set to operate with sufficient resolving powersfor each application. This resolution might typically be between 1 amuto fractions of an amu across the mass/charge ratio range chosen. Thewidth of the analysis filter's band pass determines the resolution ofthe mass spectrometer.

With reference to FIG. 2, a second embodiment is shown. Here, the massspectrometer 50 also comprises an ion beam source 12 and source vacuumpump 24, if required. However, in this embodiment the sacrificial massfilter 52 is close coupled to the analysis mass filter 54. Thus, bothfilters are disposed in a single vacuum chamber 56. This arrangementprovides improved transmission in comparison with the first embodimentshown in FIG. 1, where the sacrificial filter is separated from theanalysis filter.

Further embodiments of the apparatus might include additional filters,or the like, within the vacuum chamber system. These additionalcomponents might be particularly useful if MS-MS experiments are beingperformed. Furthermore, additional multi-pole structures may beincorporated in the instrument comprising collision/reaction cells orion guides. Auxiliary electrodes driven by AC voltages only may also beincluded to improve transmission. It may be desirable to locate theseadditional components between the sacrificial and analysis filters.

Other multipole arrangements, besides quadrupoles, can be used to filterions outside a mass/charge ratio from the ion beam and preferably theanalysis and sacrificial filters have the same rod configuration, butnot necessarily rod length. If resolving powers below 1 amu arerequired, it is preferable to configure the rods in a quadrupolearrangement.

The opposing rods of the filters (in a quadrupole configuration) arespaced apart by a distance 2r₀. Preferably, r₀ for both the sacrificialand analysis filters are equal and between 1 mm and 15 mm, or morepreferably between 4 mm and 8 mm. The length of the sacrificial filterrods, L1, should be between 1 and 80 times r₀, but preferably between 2to 6 times r₀. The analysis filter rod length, L2, is preferably between20 to 80 times r₀. For high resolution applications there can be acompromise between the rod length (to maximise the time ions spend inthe filter) and engineering tolerances that constrain how long rods canbe made to a given accuracy. At the priority date of this application anoptimum length for L2 is 250 mm, where r₀=6 mm. Filter rod manufacturingmethods may improve with time, and the upper limit of 80r₀ for the rodlength should not be limiting.

Typically, the chamber length containing the sacrificial filter needonly be a few percent longer than the filter rods, although it can belonger to accommodate additional components.

Preferably the DC bias (pole bias) applied to all the rods in thesacrificial filter is controlled independently to the pole bias of theanalysis filter rods. In this way, the kinetic energy of the ions ineach filter can be controlled independently, for the reasons previouslydescribed.

Also, it is preferable to connect the sacrificial filter, via an RFcoupler such as capacitors, to the analysis filter's power supply. Thus,the sacrificial filter has the same RF voltage as the analysis filterthereby reducing the need for additional power supplies, and hencereducing the overall cost of the instrument. In this preferredembodiment, the sacrificial filter has a different DC potential appliedto the rods compared to the analysis filter DC potential since thesacrificial filter operates at a different resolution. In the case ofthe sacrificial filter, the DC potentials require relatively lowprecision since they are applied to a low resolution mass filter.

Filter resolution can be controlled by varying the RF to DC voltageratio. For very high resolution the RF:DC ratio should lie between−5.963 and −5.958. The ratio for the sacrificial filter should liebetween −5.983 to −6.00. (The voltages are calculated using knownequations, such as equation 2.19 and 2.20 in “Quadrupole MassSpectrometry and its Applications”, by P H Dawson, published byElsevier, 1976, for example, assuming the ions transmitted have anamu=115, r₀=6.0 mm, V_(RF)=−1205.44V, V_(DC)=202.24V, and RF drivefrequency=2.0 MHz, given an RF:DC ratio of −5.96).

The filter chambers preferably operate at the same pressure and below10⁻³ mbar, and more preferably below 10⁻⁵ mbar.

In another embodiment, an auxiliary rod system, similar to the systemdisclosed in U.S. Pat. No. 3,129,327 may be utilised to improvetransmission into the sacrificial filter.

Embodiments of this invention are distinguished from other systems sincethe sacrificial filter transmits ions having substantially the samemass/charge ratio as those transmitted by the analysis filter. Otherdevices have been previously proposed to operate by selecting a parention in the first filter and where daughter ions of a differentmass/charge ratio are transmitted by the second filter.

In the preferred embodiments the analysis filter determines theresolving power of the spectrometer. A spectrum of the ion beam can beproduced by scanning the band pass of the filters through the desiredrange of mass/charge ratios. It is preferable to scan both filters atthe same time to produce the spectrum. The scan can be a smooth scanthrough a range of mass/charge ratios or a jump scan where both thefilter's transmission characteristics are stepped from one transmissionpeak to another. The jump scan can be particularly useful if areas ofthe spectrum are of no interest to the end-user.

Since both filter's transmission profiles are likely to be non-uniform(that is, the transmission does not have a ‘top-hat’ like profile) it isimportant to scan both the sacrificial and analysis filter together. Inthis way, any substantial modulation of the spectrum can be minimised.In a preferred embodiment, the filter's transmission profiles arescanned across the desired range of mass/charge ratios by scanning thepower supply to the filters.

The RF:DC ratio determines the band pass width of the mass filters andso the analysis filter has a different RF:DC ratio applied compared tothe sacrificial filter. A change to the rod voltage amplitude changesthe mass/charge ratios transmitted through the filter. So, to achieve ascan through a mass/charge range, the analysis filter's supply isincreased in amplitude, but the RF:DC ratio remains constant throughoutthe amplitude increase. If the sacrificial filter's RF supply is coupledto the analysis filter (as described above), then the RF signal strengthon the sacrificial filter is also modulated. Thus, the sacrificialfilter's separate DC supply should be modulated to scan the sacrificialfilter through the mass/charge range whilst keeping its RF:DC constant.The sacrificial filter's DC supply is ramped up using a separate scannerdevice, since the sacrificial filter has a separate DC supply in thepreferred embodiment. In this way, both the filter's transmissioncharacteristics are scanned through the mass/charge range of interestwithout moving relative to one another (that is, the rate at which thefilters are scanned over the mass/charge ratio is substantially the samefor both filters).

If the filter transmission profiles are known, it may be desirable toscan the analysis filter only through the range transmitted by thesacrificial filter, particularly if the spectrum range is within theband pass of the sacrificial filter. However, a compensation factorshould be added to the detected spectrum to compensate for the uneventransmission profile. If the spectral range is broader than thesacrificial filter's band pass, then both filters may have to bescanned. In which case, the sacrificial filter can be scanned coarselywhilst the analysis filter is scanned finely to produce the spectrum.

The detector and scan controller are preferably computer controlled,thereby allowing the capture of the spectrum to be automated. Suitabledetectors and scan controlling means are known in the art.

Although FIGS. 1 and 2 show the filters on a common axis, it may bedesirable to arrange the analysis filter pff-axis to the sacrificialfilter. As a result, there would be no line-of-sight path from thesacrificial filter to the detector, through the analysis filter. Thishas the advantage of reducing the background count rate of the detector.Such a background count may be as a result of neutral species passingthrough the filter system. Of course, the skilled person appreciatesthat neutral species are not affected by the filters quadrupole fieldand thus pass straight through the filter. There are several ways todisplace the axis of the sacrificial and analysis filter from oneanother including disposing a different ion optical device between thetwo filters. An alternative arrangement would be to arrange the axis ofthe sacrificial filter so that it intersects the axis of the analysisfilter at an angle to, and substantially at the entrance of, theanalysis filter stage.

Further embodiments within the scope of the invention will be envisagedby the skilled person. For example, it may be desirable to have two ormore analysis or sacrificial filters to further improve performancecharacteristics of a mass spectrometer. Also, other components might bedisposed in series and between the sacrificial filter and the analysisfilter; the two mass filters do not have to be juxtaposed. Of course,this invention is not limited to quadrupole mass filter configurations.Other configurations of filter can be used in embodiments within thescope of this invention.

1. Mass filter apparatus for filtering a beam of ions having mass/chargeratios in a range of mass/charge ratios to transmit ions of a selectedmass/charge ratio in the said range, comprising an ion beam source foremitting the ion beam, first and second mass filter stages in series toreceive the beam from the beam source, and a vacuum system formaintaining at least the second filter stage at an operating pressurebelow 10⁻³ torr, wherein said vacuum system is arranged to maintain boththe first and second filter stages at operating pressures below 10⁻³torr, the first mass filter stage is arranged for transmitting only ionshaving a sub-range of mass/charge ratios which includes the selectedmass/charge ratio, and the second mass filter is arranged fortransmitting only ions of the said selected mass/charge ratio.
 2. Anapparatus according to claim 1, wherein the first mass filter stage isarranged to have a broader band pass characteristic compared to thesecond mass filter stage.
 3. An apparatus according to claim 1, whereinthe ions within the sub-range comprise 1%, or less, of the ions withinthe beam.
 4. An apparatus according to claim 1, wherein the ions withinthe sub-range comprise 0.01%, or less, of the ions within the beam. 5.An apparatus according to claim 1, wherein each filter stage comprises amulti-pole analyser.
 6. An apparatus according to claim 5, wherein eachfilter stage comprises rods in a quadrupole arrangement.
 7. An apparatusaccording to claim 5, further comprising a DC and AC voltage supply forapplying a driver voltage to the rods of each filter stage.
 8. Anapparatus according to claim 5, wherein an AC voltage supply isconnected to one of the filter stages and another filter stage iselectrically coupled to the one filter stage by an RF coupler.
 9. Anapparatus according to claim 1, further comprising a scanner forcontrolling at least the second filter stage so that the mass/chargeratio of transmitted ions is scanned over a scanned range to provide amass spectrum.
 10. An apparatus according to claim 9, wherein thescanner is arranged to control also the first filter stage so that acentre point of the sub-range of mass/charge ratios transmitted by saidfirst filter stage substantially tracks the scanned mass/charge ratiotransmitted by the second filter stage.
 11. An apparatus according toclaim 1, wherein the first filter stage is arranged off axis withrespect to the second filter stage.
 12. An apparatus according to claim11, wherein the longitudinal axis of the first filter stage is arrangedto intersect with the longitudinal axis of the second filter stagesubstantially at the end of the second filter stage nearest to the firstfilter stage.
 13. Mass spectrometer comprising a mass filter apparatusaccording to claim
 1. 14. A method for filtering a beam of ions havingmass/charge ratios within a range of mass/charge ratios to transmit ionsof a selected mass/charge ratio in the said range, the methodcomprising; emitting the ion beam from a beam source into a first massfilter stage; transmitting through the first mass filter stage only ionshaving a sub-range of mass/charge ratios which includes the selectedmass/charge ratio; and transmitting through a second mass filter stagein series with the first mass filter only ions having the selectedmass/charge ratio, wherein the first and second filter stages operate atpressures below 10⁻³ torr.
 15. A method according to claim 14, whereinthe ions within the sub-range comprise 1%, or less, of the ions withinthe beam.
 16. A method according to claim 14, wherein the ions withinthe sub-range comprise 0.01%, or less, of the ions within the beam. 17.A method according to claim 14, wherein each filter stage comprises amulti-pole mass filter, and a DC and AC driver voltage is applied to thefilter.
 18. A method according to claim 17, wherein an AC voltage issupplied to one filter stage and another filter stage is electricallycoupled to the first filter stage by an RF coupler.
 19. A methodaccording to claim 14, wherein a vacuum system maintains at least thesecond stage at an operating pressure below 10⁻³ torr, and both thefirst and second stages are maintained at an operating pressure below10⁻³ torr.
 20. A method for producing a mass spectrum of an ion beamhaving mass/charge ratios within a range of mass/charge ratios,comprising; emitting the ion beam from a beam source into a first massfilter stage, transmitting only ions having a sub-range of mass/chargeratios which includes a selected mass/charge ratio through the firstmass filter, transmitting only ions having the selected mass/chargeratio through a second mass filter in series with the first mass filterto a detector for detecting any ions having the selected mass/chargeratio, controlling at least the second filter stage so that themass/charge ratio of transmitted ions is scanned over a scanned range,and detecting the number of ions transmitted by the second filter stageat any given mass/charge ratio to provide a mass spectrum, wherein thefirst and second filter stages operate at pressures below 10⁻³ torr. 21.A method according to claim 20, further comprising controlling themass/charge of ions transmitted by the first filter stage so that acentre point of the sub-range of mass/charge ratios transmitted by saidfirst filter stage substantially tracks the scanned mass/charge ratiotransmitted by the second filter stage.
 22. A method according to claim20, wherein the ions within the sub-range comprise 1%, or less, of theions within the beam.
 23. A method according to claim 20, wherein theions within the sub-range comprise 0.01%, or less, of the ions withinthe beam.
 24. A method according to claim 20, wherein each filter stagecomprises a multi-pole mass filter, and a DC and AC driver voltage isapplied to the filter.
 25. A method according to claim 24, wherein an ACvoltage is supplied to one filter stage and another filter stage iselectrically coupled to the first filter stage by an RF coupler.
 26. Amethod according to claim 24, wherein a scanner controls the AC and DCvoltage amplitudes over a voltage range, and the AC:DC voltage ratioconstant is kept substantially constant.
 27. A method according to claim20, wherein a vacuum system maintains at least the second stage at anoperating pressure below 10⁻³ torr, and both the first and second stagesare maintained at an operating pressure below 10⁻³ torr.
 28. A methodfor filtering ions with a given mass/charge ratio from a beam of ionshaving an array of mass/charge ratios, in a mass spectrometer comprisingan ion beam source for emitting the ion beam, a detector or output fordetecting or transmitting the filtered ions, and a plurality of massfilters disposed in series between the beam source and the detector oroutput, the filters having the same operating pressures at or below 10⁻³torr, the method comprising; emitting the ion beam from a beam sourceinto a first mass filter, transmitting only ions having a range ofmass/charge ratios which includes the mass/charge ratio of the filteredions from a first mass filter, and transmitting only the filtered ionsfrom a second mass filter, disposed between the first mass filter andthe detector or output.
 29. A method according to claim 28, wherein theions within the sub-range comprise 1%, or less, of the ions within thebeam.
 30. A method according to claim 28, wherein the ions within thesub-range comprise 0.01%, or less, of the ions within the beam.
 31. Amethod according to claim 28, wherein the first and second filter stagesoperate at pressures below 10⁻³ torr.
 32. A method of improving theresolving power of a mass spectrometer, comprising; emitting an ion beamfrom a beam source into a first and second mass filter stages in series,the ions in the beam having mass/charge ratios within a range ofmass/charge ratios; transmitting through the first mass filter stageonly ions having a sub-range of mass/charge ratios which includes aselected mass/charge ratio; receiving only ions in said sub-range at thesecond filter stage; transmitting through a second mass filter stageonly ions having the selected mass/charge ratio, whereby the secondfilter stage can operate with reduced ion beam current.
 33. A methodaccording to claim 32, wherein the ions within the sub-range comprise1%, or less, of the ions within the beam.
 34. A method according toclaim 32, wherein the ions within the sub-range comprise 0.01%, or less,of the ions within the beam.
 35. A method according to claim 32, whereinthe first and second filter stages operate at pressures below 10⁻³ torr.36. A method for reducing the deposition of material on multipoleelements of a primary resolving filter of a mass spectrometer,comprising emitting an ion beam from a beam source into a first massfilter stage, the ions in the beam having mass/charge ratios within arange of mass/charge ratios, transmitting through the first mass filterstage only ions having a sub-range of mass/charge ratios which includesa selected mass/charge ratio, receiving only ions in said sub-range at asecond filter stage in series with said first filter stage, said secondfilter stage constituting said primary resolving filter, andtransmitting through the second filter stage only ions having a selectedmass/charge ratio within the sub-range, thereby reducing the number ofions rejected in said primary resolving filter.
 37. A method accordingto claim 36, wherein the ions within the sub-range comprise 1%, or less,of the ions within the beam.
 38. A method according to claim 36, whereinthe ions within the sub-range comprise 0.01%, or less, of the ionswithin the beam.
 39. A method according to claim 36, wherein the firstand second filter stages operate at pressures below 10⁻³ torr.